COVID-19 illness, antibodies and testing

The serological response to SARS-2 virus is individual. In general, the antibody levels increase fast after the infection. Most persons with COVID-19 illness showed an antibody response between day 10 and day 21 after infection. The seropositivity was cumulatively 50% within two weeks and 100% on the 39th-day.

The decrease is also individual, and in addition related to symptoms. 40% out of those individuals who became asymptomatic became also negative for IgG in the early convalescent phase, but only approx. 13% out of the symptomatic became seronegative.

The serological tests can be offered for patients in addition to direct detection methods if they have been symptomatic for more than nine days. They benefit from the serological tests since the sensitivity of direct virus detection decreases and the serological tests’ sensitivity increases over time.

Sources:

1. Immune responses and immunity to SARS-CoV-2 https://www.ecdc.europa.eu/en/covid-19/latest-evidence/immune-responses
2. Zhao J, Yuan Q, Wang H, Liu W, Liao X, Su Y, et al. Antibody responses to SARS-CoV-2 in patients of novel coronavirus disease 2019. Clinical Infectious Diseases. 2020. DOI: 10.1093/cid/ciaa344
3. Clinical and immunological assessment of asymptomatic SARS-CoV-2 infections https://doi.org/10.1038/s41591-020-0965-6
4. Interim Guidelines for COVID-19 Antibody Testing in Clinical and Public Health Settings https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antibody-tests-guidelines.html